Secret to Squid's Iridescent Rainbow Skin Discovered

Below:

Next story in Science

For squid looking to sparkle, extra bling is only seconds away,
thanks to a nerve network in the skin that allows these
cephalopods to alter their iridescence — the first invertebrate
creatures found to have this ability.

A new study finds that electrical stimulation of the nerves in
squid skin changes the color and reflectance of tiny platelike
structures called iridophores in the skin, allowing changes in
hue from red all the way through the color spectrum to blue.

Oddly enough, despite their bright displays, these squid see only
in black-and-white, deepening the mystery of why and how they
pick a color from their array.

"The cool thing about it is that these animals
are colorblind and yet they are producing a color signal,"
said study researcher Paloma Gonzalez Bellido of the Marine
Biological Laboratory (MBL) at Woods Hole, Mass. "It's puzzling
to us — even if it isn't for [squid to see], if it is for
camouflage, how do you know you're doing this right? You can't
see color." [ See
Video of the Squid Color Changes ]

Unlike most colors we see, which are caused by pigments absorbing
and reflecting certain wavelengths of light, iridescence
is caused by structures interfering with the reflectance of
light, causing the wavelengths to interact with one another and
creating intense, almost metallic hues. Iridophores are made of
complex stacked plates that cause this interference, Wardill told
LiveScience.

What wasn't clear is how the iridophores worked. By definition,
iridescent color appears slightly different when viewed from
different angles, Wardill said, so measuring iridescence changes
is tricky.

To figure out the iridophores' secrets, the researchers carefully
dissected the skin of dead longfin inshore
squid (Doryteuthis pealeii). They traced the nerves
of the skin and stimulated them electrically, finding that they
could instigate progressive changes in skin color from the
at-rest reddish state all the way through the color spectrum to
blue.

Unlike the very fast changes seen in chromatophores, the
alteration in iridophores moves more slowly, Wardill said,
cycling through the rainbow from red to orange to yellow to green
to blue over a period of about 15 seconds.

Color-changing mystery

The neural control for the color changes isn't a local reflex,
Gonzalez Bellido said; it comes from the central nervous system.
The next mystery to solve is how precisely the squid can pick and
hold any given color, Wardill said. In the end, the researchers
hope to understand how these cephalopods decide without the
benefit of color vision what hues they need to display.

"The animals are actually
developing color on the skin and they're doing it without
pigments, and they potentially have the chance to be picking a
certain color," Wardill said. "That would be very exciting,
because there are not many examples from any animals that could
pick a color and put it on so quickly."

The researchers report their work today (Aug. 14) in the journal
Biological Sciences.